This invention relates to method for coating or coating and coloring organic or inorganic particles and application designs of the coated particles. The method is not sensitive to the nature of the particles to be coated, and therefore could be widely used for various kinds of particles, such as stone gravel, rubber fragments and plastic fragments. The invention also relates to the production of articles using colored or coated particles, for example, as binderless molding material, as substitutes for colored EPDM granules and as substitutes for gravel.
Many materials in the form of particles or fragments are widely used in current industries and in some cases, such as the case of recycled tire granules, a color that is different from and more attractive than the original color of the particles/fragments is required by marketing. Besides, the coating on the particles, whether the coating contain colorant or not, will change the physical and chemical properties (such as melting or softening temperature) of the surface of the particles/fragments.
There is a technical contradiction in coloring or coating inorganic or polymeric particles. It is clear that any relatively static contact between coated particles during the period between the time they are freshly coated and the time that the coating loses its fluidity and becomes tacky-free will result in the coated particles' sticking together and forming chunks after the coating is cured. Therefore, if the relatively static contact between coated particles can not be completely avoided, the coating should be thin enough to avoid forming strong chunks of coated particles after the coating is cured, otherwise the strong chunks can not be broken without coating damage. On the other hand, a relative thick coating is needed in some cases. For instance, when coloring dark particles (such as tire fragments) for getting a light or bright color, the coating on inorganic or polymeric particles should be thick enough to hide the dark original color of the particles and show a required bright color.
The current commercial coating or coloring processes, which are represented by those described in U.S. Pat. Nos. 6,036,998 and 5,714,263, usually have two common points: (1) They need separate curing or drying equipment such as a heating tunnel to cure the coating after finishing mixing process in a mixer; (2) The freshly coated particles have a chance to stick to each other on a conveyer that carries them through the heat curing equipment. Even if using a vibrating conveyer, the freshly coated particles still risk sticking to each other when the particle size is fine (such as 0.5-3 mm). A separate curing step and the required heating unit increase equipment investment, processing time, energy and labor cost. The relatively static contact between coated particles during the curing period will cause particle chunks to be formed after cure. In order to avoid forming strong chunks that can not be broken without coating damage, the color coating has to be made thin enough not to bind strongly, and, therefore, in many cases, a satisfactory color shade cannot be obtained.
Recycled polymer fragments are currently widely used to make molded products, such as rubbery tiles and sheets. However, due to the thermosetting character of many recycled polymer particles (such as tire fragments or thermosetting plastic fragments), the manufacturers of these molded products have to use a liquid resin system as binder to hold the particles and form a final product with designed shape. The using of binder complicates the required processing equipment that must include two mixing units—one for mixing a formulated binder system and another one for mixing binder and particles. Additionally, the liquid binders are usually toxic chemicals (such as urethane prepolymers). Furthermore, the mixing units have to be cleaned every time after use and that costs much labor expense.